Wireless broadband services can provide tens of megabits per second of capacity per channel from each base station (BS) to subscriber stations (SS) such as mobile stations (MS) with a baseline configuration. The high data throughput (peak data rate) is essential requirement to enable broadband data services including data, streaming video and voice-over-IP (VoIP) with high quality of service. Wireless broadband technologies have been developed to provide broadband wireless access in order to adapt services and applications for mobile Internet.
The transmission data rate in the wireless link between BS and MS depends on the channel conditions that may vary substantially during the link connection. Coding and modulation schemes used for transmission of data over the air interface have an effect on transmission data rate.
Technical Specifications IEEE 802-16e (Mobile WiMax) describes Orthogonal Frequency Division Multiplexing (OFDM) multiplexing technique that subdivides the bandwidth into multiple frequency sub-carriers where the input data stream is divided into several parallel sub-streams of reduced data rate and each sub-stream is modulated and transmitted on a separate orthogonal sub-carrier. The reduced data rate means increased symbol duration and therefore improves the robustness of OFDM to delay spread. OFDM exploits the frequency diversity of the multipath channel coding and interleaving the information across the sub-carriers prior to transmission. In OFDM resources are available in the time domain by means of OFDM symbols and in the frequency domain by means of sub-carriers. The time and frequency resources can be organised into sub-channels for allocation to individual users. Active sub-carriers are grouped into sub-channels for both DL and UL transmission.
According to the WiMax specifications MS can feedback channel-state information. A channel quality indicator (CQI) channel is utilized to provide channel-state information from the user terminals to the base station scheduler. Relevant channel-state information includes physical CINR (carrier-to-interference-and-noise ratio), effective CINR, MIMO (multiple-input multiple-output) mode selection and frequency selective sub-channel selection. The CQI channel provides channel information feedback to enable scheduler to choose appropriate coding and modulation for each allocation. CQI contains measurement feedback information for BS to select transport format (MCS—modulation and coding scheme) and resource. The adaptive modulation and coding, HARQ (hybrid auto repeate request) and power control provide robust transmission over the time-varying channel. Since the resource allocation information is conveyed in the MAP message at the beginning of each frame, the scheduler can change the resource allocation on a frame-by-frame basis in response to traffic and channel conditions.
Mobile WiMax specifications describe antenna technologies that support two MIMO modulation schemes: space-time code (STC) and spatial multiplexing (SM). STC provides large coverage area regardless of channel condition but does not improve the peak data rate (throughput). With 2×2 MIMO, SM increases the peak data rate (throughput) by transmitting two data streams when channel conditions are adequately good. Mobile WIMAX supports switching between these options under different channel conditions but the selection does not guarantee the best transmission data rate in every channel conditions for the reasons below.
The disadvantage of known spatial modulation mode selection mechanisms is that BS has to rely on capabilities offered by MSs. The measurement of the channel condition of the wireless downlink is typically MS related procedure. MS is arranged to measure channel quality e.g. using average CINR (carrier-to-interface-and-noise ratio) measurements. The average CINR measurement by the MS can be used as a basis for selecting the preferred spatial mode to be used by BS. However, if BS relies on this preference it may not be able to make always correct decisions for optimal transmission mode in instantaneous channel conditions. This is the case when MS is not supporting modulation mode selection feedback measurements or when MS is not capable of signalling its modulation mode preference itself. Some of known spatial mode selection mechanisms rely on reciprocity of the wireless channel between downlink (DL) and uplink (UL) which is not always satisfied. Some known mechanisms require complex calculation of eigen-value decomposition of the channel matrix, and they are therefore complex solutions. Technical documents in UMTS (universal mobile telecommunications system) specification for one suggests that the mode change is solely based on the SNR (signal-to-noise ratio) so that the users close to BS employ spatial mode and other users use diversity. Also when MS goes around a corner of the building and thus moves from the line-of-sight (LOS) area to non-line-of-sight (NLOS) area, the SNR and channel capacity goes down dramatically whereas the delay spread goes up significantly during the LOS-NLOS change. The reason for the capacity drop is the fact that the signal level (and the SNR) is greatly reduced in NLOS area. The above-mentioned spatial mode selection mechanisms do not support BS sufficiently accurate so that it can make a correct decision of mode selection for optimal transmission data rate in instantaneous channel conditions.
For the reasons above the optimal capacity of the channel, i.e. the best transmission mode in the wireless link between BS and MSs is not always guaranteed. The problems set forth above are overcome by centralizing a decision making of the transmission mode in BS.